DE69220293T2 - HYDRAULIC CONTROL SYSTEM FOR HYDRAULIC EARTH CONSTRUCTION MACHINE - Google Patents

Description

The invention relates to a hydraulic control system for a hydraulic construction machine with: a hydraulic pump with adjustable displacement or adjustable hydraulic pump driven by a prime mover; a first hydraulic actuator which is driven by oil discharged from the hydraulic pump; a first control valve which is provided between the hydraulic pump and the first hydraulic actuator and serves to control the supply of pressurized oil to the first hydraulic actuator; a first determining device for determining a first target displacement in order to keep a discharge pressure of the hydraulic pump by a predetermined target value at a value above a load pressure of the first hydraulic actuator; a second determining device for determining a second target displacement, by which an input torque is limited based on the discharge pressure of the hydraulic pump; and a displacement control device for controlling the displacement of the hydraulic pump such that the displacement is equal to the first and/or the second target displacement, the hydraulic control system further comprising a limiting device with which a limit can be imposed on a signal indicating the first target displacement determined and output by the first determining device.

A hydraulic control system for a hydraulic construction machine of the above-mentioned type is disclosed in EP-A-0 379 595.

A hydraulic wheel excavator such as that shown in Fig. 10 is known as a hydraulic construction machine equipped with the above type of hydraulic control system. In the figure, reference numeral 4 denotes a hydraulic motor for vehicle propulsion, and rear wheels 103 are driven by the rotation of the hydraulic motor 4 via a transmission 101 and a drive shaft 102 to enable the vehicle to travel. A boom 104 serving as a part of a front attachment is moved up and down by an extension and contraction operation of a boom cylinder 21. In this specification, an excavation work and the like using the front attachment is described. while the vehicle is not moving is simply called "work", and as a distinction there is a term "driving".

As one of the hydraulic construction machines as described above, there has been proposed a machine in which load measurement control and input torque limiting control are selectively carried out (for example, as described in Japanese Patent Application Laid-Open No. 2-118203). These control systems will be described below with reference to Fig. 11.

Fig. 11 shows a hydraulic circuit for traveling and working of the hydraulic excavator described above, and reference numeral 1 represents a variable displacement hydraulic pump or adjustable hydraulic pump driven by a diesel engine 27. The rotation speed of the engine 27 is controlled by a rotary member of a governor lever 27b of a governor 27a driven by a stepping motor 28 in accordance with an amount of operation of a throttle lever (not shown) or an amount of operation of an accelerator pedal 6a for traveling (hereinafter referred to as an accelerator pedal). A variable displacement hydraulic pump 1 discharges oil whose flow rate corresponds to the engine speed and the displacement thereof. The discharged oil is supplied to the hydraulic motor 4 via a traveling control valve 2 and further to a boom drive hydraulic cylinder 21 via a working control valve 20.

The load sensing control is characterized in that the displacement (hereinafter also referred to as "tilt angle") of the adjustable hydraulic pump 1 is controlled so that the differential pressure between front and rear stages of the travel control valve 2 or the working control valve 20, i.e. the differential pressure between a pressure (pump pressure) at an inlet port of the control valve 2, 20 and a pressure (load sensing pressure) at an outlet port thereof is set to a constant value, thereby keeping the pump pressure at a value above the load sensing pressure by a predetermined set value. The load sensing pressure is a pressure selected by a shuttle valve 29 and the higher of load pressures of the hydraulic motor 4 and the hydraulic cylinder 21.

The system for carrying out the load measuring control, as shown in Fig. 11, is equipped with a load measuring control device 11, the switching operations of which are carried out in accordance with the differential pressure between the pump pressure and the load measuring pressure. When the differential pressure between the pump pressure and the load sensing pressure exceeds a pressure set by a spring 11a, the load sensing control device 11 is switched to a position b in accordance with the pressure. In the position b, the pump pressure is passed to an actuating cylinder 12, and the displacement of the hydraulic pump 1 is reduced by an extension operation of the actuating cylinder 12, so that the discharge flow rate of the pump is reduced. Conversely, when the differential pressure falls to a value below the predetermined pressure, the load sensing control device 11 is switched to a position a, and the actuating cylinder 12 is connected to a tank. Consequently, the displacement is increased as described above, and thus the pump flow rate is also increased. In the system using the load sensing control, by the above operation, the tilt angle of the pump is controlled so that the pump flow rate is equal to a required flow rate of the control valve 2 or 20, so that the discharge of an excess flow rate of oil can be prevented and the useless use of oil due to diaphragm leakage can be prevented to improve the fuel consumption and the operating performance.

The functionality of the drive circuit is described below.

When a forward/reverse travel switching valve 8 is switched to a forward travel position (F position) and the pedal 6a of the servo valve 6 is depressed, oil discharged from the hydraulic pump 5 is supplied to a servo port 2a of the servo type control valve 2 so that the control valve 2 performs its switching operation by a stroke amount corresponding to a servo oil pressure. By this operation, oil discharged from the variable displacement hydraulic pump 1 is supplied to the hydraulic motor 4 via a pipe 91, a pressure compensation valve 23, the control valve 2, a pipe 92 and a compensation valve 3 to enable the vehicle to travel. In addition, the engine speed is controlled in accordance with the amount of depression of the accelerator pedal 6a so that the traveling speed of the vehicle depends on the amount of depression of the accelerator pedal 6a.

When the accelerator pedal is released while the vehicle is running, the communication between the outlet port and the inlet port of the servo valve 6 is interrupted, and the outlet port of the servo valve 6 is communicated with a tank port connected to the tank 10. As a result, the pressure oil acting on the servo port 2a flows through the forward/reverse travel switching valve 8, a slow return valve 7 and the servo valve 6 to tank 10. At this time, the return oil is throttled by an orifice 7a of the slow return valve 7, so that the vehicle is gradually braked while the servo type control valve 2 is gradually switched to a middle valve position.

On the other hand, the operation of the working circuit is as described below.

When the work switching valve 20 is switched to a "b" position or a "c" position by a hand lever 20a, the oil discharged from the hydraulic pump 1 is supplied to the boom drive hydraulic cylinder 21 via the pipe 91, the pressure compensating valve 24, a line 94 and the control valve 20, and the boom 104 is moved up and down as shown in Fig. 10 by the extension and contraction operation of the hydraulic cylinder 21. The pressure compensating valves 23 and 24 enable the hydraulic pump 1 to supply the hydraulic motor 4 and the hydraulic cylinder 21 with pressures each higher than the load pressures of them by a predetermined pressure, so that both actuators can be operated independently of each other.

The hydraulic control system, as shown in Fig. 11, is equipped with a torque control servo valve 25 for carrying out the input torque limiting control, and the servo valve 25 is supplied with the pressure output from the hydraulic pump 1 as a servo pressure. When the servo pressure acting on the servo valve 25 exceeds a pressure set by a torque limiting setting spring 25a, the servo valve 25 is switched from the c position to the d position as shown in the figure. By this operation, the pressure output from the hydraulic pump 1 acts on the servo cylinder 12, thereby reducing the displacement of the hydraulic pump 1, and the torque of the hydraulic pump 1 is kept within a range of the output torque of the engine 27, thereby preventing an overload from acting on the engine 27. This operation is called input torque limiting control.

According to the hydraulic control system thus constructed, the displacement of the hydraulic pump is controlled to be as large as the smaller value of the target displacement by the load sensing control (first target displacement) and the target displacement by the input torque limited control (second target displacement), and by this operation, an improvement in fuel consumption and operating performance and prevention of overloading of the engine 27 are achieved. Reference numeral 26 represents a relief valve operated by the differential pressure between the pump pressure and the load sensing pressure, reference numerals 31a and 31b represent a cross-relief valve, and CJ represents a central junction.

The hydraulic control system equipped with the load sensing control and input torque limiting systems is constructed so that the maximum value of the displacement of the variable displacement hydraulic pump is limited by a value set on the input torque limiting control system side by the control valve 25, and in a range below this maximum value, the displacement of the variable displacement hydraulic pump is fixed to the displacement set on the load sensing control system side by the load sensing controller 11. As a result, the displacement of the variable displacement hydraulic pump 1 is controlled to be set consistently to the input torque limiting control value or the load sensing control value based on the amount of operation of the accelerator pedal, the amount of operation of the hand lever, or the circuit pressure condition, regardless of a change in the operating circumstance such as "driving" or "working" or an operator's preference.

Therefore, the following two problems occur

(1)

In a neutral state in which the operating lever 20a and the accelerator pedal 6a are not operated, the load sensing control device 11 and the control valve 25 are switched to the b position and the c position, respectively, to limit the displacement of the variable displacement hydraulic pump 1 to the minimum value, and thus the pump discharge rate is low. Therefore, when the operating lever is quickly operated from the neutral position, even if an increase in the pump displacement is intended in accordance with the amount of operation of the operating lever, a long time is required until the displacement reaches a displacement corresponding to the operating lever position. Therefore, there is a problem that the response speed of the operating device is low. In this specification, a pump discharge rate when the control valves 22 are in their neutral position is referred to as a "standby discharge rate".

(2)

The applicant of the present application has proposed an apparatus in which the engine speed is controlled in accordance with the amount of operation of the accelerator pedal 6a in the working operation, as disclosed in Japanese Patent Application Laid-Open No. 3-234364. The following problem occurs when this apparatus is used together with the hydraulic control system shown in Fig. 11 in which the displacement is controlled by selecting either the displacement set by the load sensing control or the displacement set by the input torque limiting control.

In the load sensing control in which the differential pressure at the front and rear stages of the control valve 2 or 20 is fixed to a constant value, if the opening area of the control valve 2, i.e., the stroke of the operating lever, is constant, the displacement of the hydraulic pump 1 is automatically reduced so as to prevent the increase of the differential pressure as described above even if the rotational speed of the engine 27 is increased and thus the pump output rate is increased, but it is increased so as to prevent the drop of the differential pressure even if the rotational speed of the engine 27 is decreased and thus the pump output rate is decreased. Therefore, in the working operation, when the load sensing control is carried out with the control of the engine speed using the accelerator pedal 6a, by which no adjustment of the opening area of the control valve 2 is carried out, the pump discharge flow rate remains constant even if the engine speed is reduced, and the driving speed of the front attachment (for example, boom) is not changed, whereby the efficiency in the working operation is low. The speed of the front actuator can be controlled by the control of the engine speed in a state where a differential pressure which is set in advance does not occur even if the tilt angle of the hydraulic pump is increased to the maximum value or if a target flow rate exceeds a torque limiting flow rate (in a saturation state).

An object of this invention is to improve the known system and to provide a hydraulic control system for a hydraulic construction machine in which a setpoint value of a displacement of a pump based on a load measuring control and a setpoint value of the displacement of the pump based on an input torque limiting control are not selected to be constant, in particular the setpoint value of the displacement of the pump based on the load sensing control is limited, and the selected setpoint differs between cases where a limitation is imposed and those where no limitation is imposed, even for the same operating condition.

The invention provides for this purpose a system according to the preamble of claim 1, which is characterized in that the hydraulic control system further includes a limiting device with which a limit can be imposed on a signal indicative of the target displacement determined and output by the first determining device, the limiting device including: a limit signal output device for outputting a limit signal indicating a value which is greater than the minimum value of the first target displacement output by the first determining device, a maximum value selection device for comparing the value indicated by the limit signal with the first target displacement output by the first determining device and for selecting and outputting the larger of these values, and a limit operation instruction signal output device for outputting a signal instructing a limit operation which causes the operation of the limit signal output device, and the displacement is controlled so that that it is as large as the smaller of the target displacement selected by the maximum value selection device and the second target displacement.

According to a second embodiment, the invention provides a system according to the preamble of claim 10, characterized in that the hydraulic control system further includes a limiting device with which a limit can be imposed on a signal indicating the first target displacement determined and output by the first determining device, the limiting device including: a minimum value selection device for selecting the smaller of the first target displacement output by the first determining device and the second target displacement output by the second determining device, a selection device for selecting the target displacement selected by the minimum value selection device or the second target displacement, and a selection instruction signal output device for outputting a selection instruction signal which instructs the output variable the minimum value selection device or the second target displacement as the target displacement to be selected in the selection device.

The system according to claim 1 is applicable to a hydraulic construction machine comprising: a variable displacement hydraulic pump driven by an engine; a first hydraulic actuator driven by oil discharged from the hydraulic pump; a first control valve provided between the hydraulic pump and the first hydraulic actuator and serving to control the supply of pressurized oil to the first hydraulic actuator; a first determining device for determining a first target displacement for setting a discharge pressure of the hydraulic pump by a predetermined target value to a value above a load pressure of the first hydraulic actuator, a second determining device for determining a second target displacement by which an input torque is limited based on the discharge pressure of the hydraulic pump; and a displacement control device for controlling the displacement of the hydraulic pump such that the displacement is equal to the first and/or the second target displacement.

The above-mentioned object is achieved by providing a control device with which a limitation can be imposed on a signal representing the first target displacement determined by the first determining device and output.

The displacement of the variable displacement hydraulic pump is controlled to equal the target displacement determined by the first determining means or the target displacement determined by the second determining means. Through a mode of the control means, the displacement of the variable displacement hydraulic pump is controlled to equal a value achieved by imposing the limitation on the first target displacement or is controlled to equal the second target displacement at all times without the effect of the first target displacement.

The control device includes: a limit signal output device for outputting a limit signal indicating a value which is larger than the minimum value of the first target displacement output by the first determining device, a maximum value selection device for comparing the value indicated by the limit signal with the first target displacement output by the first determining device and for selecting and outputting the larger of these values and a limit operation instructing signal output means for outputting a limit operation instructing signal which causes the operation of the limit signal output means by an operator's intervention, and wherein the displacement is controlled to be as large as the smaller of the target displacement selected by the maximum value selecting means and the second target displacement.

For example, when the first control valve is in a neutral position and the hydraulic actuator is stopped, the first target displacement is the minimum value and the second target displacement is substantially the maximum value. Under these circumstances, when the limit operation instruction signal is output by an operator's operation, the target displacement indicated by the limit signal is selected by the maximum value selecting means and the minimum value selecting means, and the displacement of the adjustable hydraulic pump is set to a value larger than the first target displacement but smaller than the second target displacement. Therefore, the discharge flow rate of the hydraulic pump when the first control valve is in the neutral position (hereinafter referred to as "standby flow rate") and the time required for the discharge flow rate to reach a required discharge flow rate when the first control valve is rapidly operated can be shortened.

According to claim 4, the limit operation instruction signal output means preferably comprises a switch which is operated by the operator, and the operation as described above can be achieved by imposing the limitation on the first target displacement when the operator operates the switch with this intention.

As described in claim 5, at least two kinds of limiting operation instruction signals can be output from the limiting operation instruction signal output means, and a target displacement whose value corresponds to each of the two kinds of input limiting operation instruction signals can be output from the limiting signal output means. The standby throughput can be changed in accordance with the working conditions.

According to claim 6, the limit operation instruction signal output means may be designed to include an operation element for outputting includes at least two kinds of limit operation instruction signals in accordance with the extent of operation of the operating element by the operator.

Claim 7 discloses that the hydraulic construction machine further includes a first operating device which is operated to set the speed of the prime mover to an arbitrary speed, a second operating device which is operated to control the speed of the prime mover and returns to an initial low speed position when the operating force is released, a speed control device for controlling the speed of the prime mover in accordance with the first and second operating devices, a second hydraulic operating device which is driven by the oil discharged from the hydraulic pump, and a second control valve which is provided between the hydraulic pump and the second hydraulic operating device and serves to control the pressure oil supplied to the second hydraulic operating device. In this case, as the limit operation instruction signal output means, it is preferable to use a judging means for outputting a selection instruction signal indicating the selection of the second target displacement based on the discrimination of a state in which the first hydraulic operating means is operated while the rotational speed of the engine is controlled by the second operating means.

According to claim 8, the first determining means may comprise a known load measuring control system for detecting a pressure difference between the pressure of a connection line of the first control valve with the hydraulic pump and the pressure of a connection line of the first control valve with the first hydraulic actuator, to calculate a deviation between a predetermined target pressure difference and the detected pressure difference, and to calculate the first target displacement based on the deviation thus calculated.

According to claim 9, the second determining means may be a known input torque limiting control system for detecting a deviation between an actual rotational speed of a diesel engine serving as a prime mover and a rotational speed indicated by the position of the governor lever of the diesel engine, for calculating, based on the detected deviation, a target torque at which the diesel engine is not subject to stalling and for detecting the discharge pressure of the variable hydraulic pump in order to calculate the second target displacement based on the reciprocal of the detected discharge pressure and the target torque.

The hydraulic control system according to claim 10 usually controls the displacement to be equal to the minimum value of the first and second target displacements, and when the instruction output signal is output, the displacement is controlled to be equal to the second target displacement regardless of the first target displacement. Therefore, the operating feeling can be improved by outputting a selection instruction signal under operating conditions in which the control of the displacement using the input torque limiting value is more preferable than the control of the displacement using the load measurement control value.

(11) This invention is also applicable to a hydraulic control system for a hydraulic construction machine comprising: a first operating device which is operated to set the rotational speed of the prime mover to an arbitrary rotational speed, a second operating device which is operated to control the rotational speed of the prime mover and which returns to an initial low-speed position when the operating force is released, a rotational speed control device for controlling the rotational speed of the prime mover in accordance with the first and second operating devices, an adjustable hydraulic pump which is driven by the prime mover, a first hydraulic actuating device which is driven by the oil discharged from the hydraulic pump, a second hydraulic actuating device which is driven by the oil discharged from the hydraulic pump, a first control valve which is provided between the hydraulic pump and the first hydraulic actuating device and serves to control pressure oil supplied to the first hydraulic actuating device, a second control valve which is provided between the hydraulic pump and the second hydraulic actuating device and serves to control pressure oil supplied to the second hydraulic actuating device, a first Determining means for determining a first target displacement to maintain the pressure delivered by the hydraulic pump at a value which is a predetermined target value above the load pressure of the first and second hydraulic Actuating means, a second determining means for determining a second target displacement to limit an input torque based on the pressure output from the hydraulic pump, a displacement control means for controlling the displacement of the hydraulic pump so that the displacement is at least equal to the first and second target displacements.

In addition, the hydraulic control system further comprises a judging means for outputting a decision signal based on the discrimination of a state in which the first hydraulic operating means is operated while the rotational speed of the engine is controlled by the second operating means. The above object can be achieved by structuring the hydraulic control system such that, when the discrimination signal is outputted, the displacement of the variable displacement hydraulic pump is controlled to be equal to the second target displacement regardless of the value of the first target displacement.

When it is discriminated that the first hydraulic operating device is operated with the rotational speed of the prime mover being controlled by the second operating device, the displacement of the variable displacement hydraulic pump is controlled to be equal to the second target displacement. In the hydraulic control system for carrying out the load sensing control and the input torque limiting control in accordance with an operating condition, for example, when the rotational speed of the prime mover is controlled in accordance with an operating amount of the accelerator pedal during the working operation other than during the traveling operation, the displacement of the variable displacement pump is forcibly controlled by the input torque limiting control. Therefore, the operating speed of the front attachment such as a boom can be controlled in response to the pedal operation through the control of the rotational speed of the prime mover by operating the accelerator pedal during the working operation, and thus the operating performance can be improved.

(12) The hydraulic control system preferably further comprises a minimum value selection device for selecting the smaller of the first and second target displacements, and a switching device for outputting the target displacement selected by the minimum value selection device when no decision signal is output and for outputting the second target displacement when the decision signal is output.

(13) The hydraulic control system may comprise: limit signal output means for outputting a limit signal indicating a value larger than the minimum value of the first target displacement, maximum value selection means for comparing the value of the limit signal and the first target displacement to select a larger one of them and outputting the selected value, minimum value selection means for selecting the smaller of the target displacement output from the minimum value selection means and the second target displacement, and limit operation instruction signal output means for outputting the limit operation instruction signal for operating the limit signal output means by an operation of the operator.

(14) The first hydraulic actuating device may comprise a hydraulic cylinder for a front attachment, the second hydraulic actuating device may comprise a hydraulic motor for vehicle propulsion, the first operating device may comprise a hand control element, and the second operating device may comprise a pedal-type control element.

(15) As the manual operating member, it is preferable to use an accelerator lever for adjusting the rotational speed of the engine in accordance with the operating position thereof, and as the pedal type operating member, it is preferable to use an accelerator pedal with which the opening area of the second control valve can be adjusted in accordance with the operating amount thereof.

According to this invention, when the rotation speed of the prime mover is controlled by operating the accelerator, the load sensing control or the input torque limiting control is carried out in accordance with the operating condition. On the other hand, when the rotation speed of the prime mover is controlled by operating the accelerator pedal in the working operation, the input torque limiting control is forcibly carried out. Therefore, when the rotation speed of the prime mover is controlled by operating the accelerator pedal in the working operation, the pump flow rate changes in accordance with the operation of the accelerator pedal, and the front attachment such as the boom can be controlled to a desired speed.

(16) If the hydraulic control system further comprises a locking device for locking the drive of the hydraulic actuating device, if the decision signal is issued, there is no danger of the hydraulic wheel excavator or the like moving unintentionally during operation, so that a very preferable advantage can be achieved.

(17) The speed control means may include: first target speed setting means for setting a first target speed of the engine in accordance with the operation amount of the first operating means, second target speed setting means for setting a second target speed of the engine in accordance with the operation amount of the second operating means, selection means for selecting a larger one of the first and second target speeds of the engine, and speed increasing/decreasing means for increasing or decreasing the speed of the engine so as to be equal to the selected target speed.

Fig. 1 is a block diagram showing a pump control system for a hydraulic control system according to a first embodiment of this invention.

Fig. 2 is a diagram showing the overall structure of the hydraulic control system.

Fig. 3 is an enlarged view of a portion of Fig. 2.

Fig. 4 is a block diagram showing a pump control system for a hydraulic control system according to a second embodiment of this invention.

Fig. 5 is a block diagram showing a pump control system for a hydraulic control system according to a third embodiment of this invention.

Fig. 6 is a block diagram showing an engine control system.

Fig. 7 is a flowchart for an engine speed control procedure.

Fig. 8 is a detailed block diagram showing an input torque control section in the first to third embodiments.

Fig. 9 is a detailed block diagram showing the input torque control section to solve a problem occurring in the input torque control section.

Fig. 10 is a side view of a hydraulic wheel excavator.

Fig. 11 is a diagram showing a conventional hydraulic control system.

BEST MODE FOR CARRYING OUT THE INVENTION - FIRST EMBODIMENT -

A first embodiment of this invention is described below with reference to Figs. 1 to 3.

The hydraulic control system according to the first embodiment is constructed such that the displacement is usually controlled to be set at the smaller of a first target displacement calculated in a load measuring control system and a second target displacement calculated in an input torque limiting control system, and when a higher response speed is required, a limit is imposed on the first target displacement to increase a standby flow rate, and then the displacement is controlled to be set at a smaller of a limit value larger than the first target displacement and the second target displacement.

Fig. 2 is a diagram showing the overall structure of a hydraulic control system for a hydraulic wheel excavator to which this invention is applied, and Fig. 3 is an enlarged diagram of a part of the control system. The same elements as in Fig. 11 are designated by the same reference numerals, and the difference between them will be mainly described.

In this embodiment, a tilt angle of the adjustable hydraulic pump 1, i.e., the displacement thereof is controlled by a tilt angle controller 40. The tilt angle controller 40 comprises: a hydraulic pump 41 driven by an engine 27, a pair of solenoid-operated valves 42 and 43, and an actuator cylinder 44 whose piston position is controlled by pressure oil from the hydraulic pump 41 in accordance with a switching operation of the solenoid-operated valves 42 and 43. The tilt angle of the hydraulic pump 1 is controlled in accordance with the piston position of the actuator cylinder 44. The switching operation of the pair of solenoid-operated valves 42 and 43 is controlled by a controller 50, which mainly comprises a microcomputer.

A forward/reverse travel switching valve 8A comprises a solenoid-operated type valve and is switched from an N position to an F position or R position when a forward/reverse travel switch SW1 on a travel panel is switched from an N position to an F position or an R position. A control valve 20A for "work" comprises a servo type valve, and the displacement direction and stroke amount are controlled by a servo pressure output from a pressure reducing valve in accordance with the operation amount of an operation lever 58.

SW2 represents a brake switch and is turned on when "working" by an operator's intervention, but is turned off when the vehicle is "driving". SW3 represents a response speed selector switch and is turned on when a fast mode is set to increase the standby throughput described later, but is turned off when a normal mode is set to take fuel consumption and noise into consideration. The output signals from these switches SW2 and SW3 are both supplied to the controller 50.

Unlike the prior art shown in Fig. 11, in this embodiment, an operating condition of the vehicle such as "working" or "driving" is electrically detected and the displacement of the hydraulic pump is electrically controlled. In this connection, the displacement of the hydraulic pump is controlled by supplying the signals of the switches as described above and a group of sensors as described later to the controller 50 and performing various calculations in the controller 50 to drive various types of actuators.

Reference numeral 51 represents a tilt angle sensor for detecting a tilt angle θs of the hydraulic pump 1, reference numeral 52 represents a pressure sensor for detecting a discharge pressure Pp of the hydraulic pump 1, reference numeral 53 represents a rotation speed sensor for detecting a rotation speed Nr of the engine 27, and reference numeral 54 represents a differential pressure sensor for detecting a differential pressure ΔPLS between the pressure discharged from the hydraulic pump 1 and the maximum load pressure of the actuator (the larger value of the load pressure of the hydraulic motor 4 and the load pressure of the hydraulic cylinder 21 selected by the shuttle valve). Reference numeral 55 represents a potentiometer for detecting a control rotation speed Nθ based on a displacement amount of the governor lever 27b, reference numeral 56 represents a pressure sensor for detecting a pressure Pt of the servo valve 6 in accordance with the amount of operation of the accelerator pedal 6a, and 95 a pressure switch which is turned on when a working servo pressure Pd is above a predetermined value. The detection result of each sensor and the on/off state of the pressure switch 95 are supplied to the controller 50. Reference numeral 57 represents a speed setting means for instructing a target speed X in accordance with the manual operation of the accelerator lever 57a, and the instruction signal therefrom is also supplied to the controller 50.

The control device 50 has a first control circuit unit 60 as shown in Fig. 1, which includes: a load measurement control section 61 (hereinafter referred to as "LS control section") for calculating and outputting a first target displacement θL, a torque control section 62 for calculating and outputting a second target displacement θA, a standby flow rate control section 63 for outputting a third target displacement θO instead of the first target displacement θL to increase the standby flow rate, a minimum value selection section 64 for selecting the smaller of the first or third target displacement input by the minimum value selection section 64 and the second target displacement input by the torque control section 62, and an actuation control section 65 for controlling the electrically operated valves 42 and 43. for displacement control based on the target displacement input from the minimum value selection section 64 and the actual displacement.

The LS control section 61 includes a target differential pressure generator 61a for outputting a signal corresponding to a target differential pressure ΔPLSR, a deviation calculation means for calculating a deviation Δ(PLS) between the target differential pressure ΔPLSR and the differential pressure Δ(PLS) detected by the differential pressure sensor 54, a calculation means 61c for calculating a change ΔθL of the target value based on the deviation Δ(PLS), and an integrating means 61d for integrating ΔθL to calculate the first target displacement θL for the load sensing control and outputting the calculated value.

The torque control section 62 includes a deviation calculation device 62a for calculating an excess torque ΔT based on an engine speed Nr detected by the speed sensor 53 and the control speed Nθ determined by the adjustment amount of the governor lever, a target torque calculator 62b for calculating a target torque Tpo for preventing engine stalling based on the excess torque ΔT, a throttle value calculator 62c for calculating a pump discharge pressure Pp detected by the pressure sensor 52, a θps calculator 62d for calculating a tilt angle θps by multiplying the target torque Tpo by the reciprocal 1/Pp, and a filter 62e for filtering the tilt angle θps and which serves as a time-delay element to output a second target displacement θA for the input torque limit control.

The standby flow rate control section 63 includes: a standby flow rate adjusting unit 63a for outputting a value corresponding to a third target displacement θO which is at least larger than the minimum value of the first target displacement θL calculated in the LS control section 61, a switch 63b which is closed to output the third target displacement θO when the response speed selector switch SW3 is turned on, and a maximum value selecting section 63c for selecting and outputting the larger of the first target displacement θL and the third target displacement θO.

The maximum value selection section 63c supplies the larger one of the first target displacement θL and the third target displacement θO to the minimum value selection section 64. The minimum value selection section 64 selects the smaller one of the first or third target displacement θL or eo and the second target displacement θA to supply the selected value to the positioning control section 65 as the tilt angle instruction value er.

The positioning control section 65 includes: a deviation calculation unit 65a for calculating a deviation between the selected tilt angle instruction value θr and an actual tilt angle value θs detected by the tilt angle sensor 51, and a function generator 65b for outputting an on signal to the solenoid-operated valve 42 or 43 when θs is larger than a value of a deadband, and controls the tilt angle controller 40 so that the pump tilt angle θs coincides with the tilt angle instruction value θr.

The operation of the first embodiment constructed in this way is described below.

The following process is carried out in the first control circuit section 60 of the control device 50 shown in Fig. 1.

(1) In a state where the operation lever 58 and the accelerator pedal 6a are in a non-operation state and each of the control valves 2 and 20A are in their neutral position, when the response speed selection switch SW3 is in an off state and the normal mode is selected, a selection signal SS output from the switch SW3 is in a low level state and the switch 63b is opened. Therefore, the maximum value selection section 63c selects and outputs the LS control value θL from the LS control section 61. When each of the control valves 2 and 20A are in their neutral position, the first target displacement θL calculated in the LS control section 61 becomes a value θLmin which is the minimum for the engine speed at that time. In addition, the pump discharge pressure Pp is the minimum value set by the relief valve 26, and the second target displacement calculated by the input torque control section 62 becomes the maximum value θAmax. Therefore, the minimum value selecting section 64 selects the first target displacement θLmin which is the LS control value, and the standby flow rate of the hydraulic pump 1 becomes a relatively small discharge flow rate Qs which is represented by the product of the first minimum target displacement θLmin and the engine speed.

When the operator operates the operation lever 58 in the above-described state, the load sensing control is carried out in which the pump pressure is kept above a load pressure of the hydraulic actuator by a constant differential pressure ΔPLSR. By this control operation, as the opening area of the control valve 20A increases in proportion to the amount of operation of the operation lever 58 to increase a required discharge flow rate, the displacement of the hydraulic pump 1 is increased. At this time, the pump discharge flow rate is moderately increased since the standby flow rate is relatively small.

When the load pressure is increased and the second target displacement θA calculated in the input torque control section 62 becomes smaller than the solve displacement output from the maximum value selection section 63c, the second target displacement θA is selected in the minimum value selection section 64. Therefore, the input torque of the hydraulic pump 1 is controlled to meet the Output torque of the engine does not exceed 27, and the occurrence of engine stalling can be prevented.

(2) In a state where the operating lever 58 and the accelerator pedal 6a are in a non-operation state and each of the control valves 2 and 20A is in its neutral position, when the response speed selection switch SW3 is in an on state and the fast mode is selected, the selection signal SS is in a high level state and the switch 63b is closed. Therefore, the maximum value selection section 63c is supplied with the third target displacement θO and the LS control value θL from the LS control section 61. When each of the control valves 2 and 20A is in its neutral position, the first target displacement θL calculated in the LS control section 61 becomes a value θLmin which is the minimum for the engine speed at that time. Therefore, when the third target displacement θO is set to a value above θLmin, the maximum value selecting section 63c necessarily selects and outputs the third target displacement θO at the non-operating time of the control valve.

In addition, at the non-operating time of the control valve, the second target displacement calculated in the input torque control section 62 becomes the maximum value θAmax as described above. Therefore, the minimum value selection section 64 selects the third target displacement θO set in the standby flow rate control section 63, and the standby flow rate of the hydraulic pump 1 becomes a relatively large discharge flow rate Qq (> Qs) represented by the product of the third target displacement θO and the engine speed.

When the operator operates the operation lever 58 in this state, the load measurement control calculation as described above is carried out in the LS control section 61, and the opening of the control valve 20A is increased in proportion to the operation amount of the operation lever 58 to increase the desired discharge flow rate. In accordance with the increase in the desired discharge flow rate, the first target displacement θL is increased. At this time, regardless of the operation of the operation lever 58, the displacement of the hydraulic pump 1 is fixed to the third target displacement value θO until the first target displacement θL is larger than the third target displacement θO. Therefore, when the fast mode is selected in an operating condition in which the operation lever 58 is operated quickly, the pump discharge flow rate increases with a high response speed following the operation of the operating lever 58 because the standby flow rate is relatively large.

In this state, when the second target displacement θA is larger than the third target displacement θO or the first target displacement θL, the minimum value selecting section 64 also selects the second target displacement θA, and thus the input torque of the pump is prevented from exceeding the output torque of the engine.

As described above, according to the first embodiment, when both control valves 2 and 20A are in their neutral positions, the displacement of the hydraulic pump 1, i.e., the standby flow rate, can be changed by operating the response speed selector switch SW3 so that the actuator is moderately operated in accordance with a fine operation of the operating member, thereby improving the fine operation in the normal mode in which the standby flow rate is small, whereas the actuator is quickly operated in accordance with the rapid operation of the operating member, thereby improving the response speed in the fast mode in which the standby flow rate is large.

In Fig. 1, when the third target displacement θO is set to be larger than the maximum value θAmax output from the input torque control section 62, the displacement of the pump is always controlled by the input torque control when the response speed selector switch SW3 is turned on, whereas the displacement of the pump is controlled in accordance with the operating conditions by the load sensing control or the input torque control when the switch SW is turned off.

The response speed selector switch SW3 may comprise a pressure switch which is turned on, for example, when the drive servo pressure is above a predetermined value. With this structure, the standby throughput becomes larger when the vehicle is running, and thus the response speed of starting the vehicle can be improved. Furthermore, a pressure switch which is turned on by the servo pressure for controlling actuators for which a different response speed than that of the actuator for "driving" the vehicle is required may be provided instead of the response speed selector switch SW3, and both This pressure switch as well as the response speed selector switch SW3 must be provided.

- SECOND EMBODIMENT -

Fig. 4 shows a second embodiment. In the first embodiment, the third target displacement θO for changing the standby flow rate is a fixed value. However, in the second embodiment, the third target displacement θO may be set to an arbitrary value in accordance with an operation amount of an operating member made by the operator.

The point different from the first embodiment is mainly described below.

A response selection dial DI such as an output intensity of an electric signal in accordance with the amount of rotation operation thereof is provided in place of the response speed selection switch SW3, and the standby throughput control section 63A is provided with a function generator 63d for outputting a third target displacement 60 in accordance with the output voltage of the dial DI in place of the setting unit 63a and the switch 63b.

With this structure, the standby throughput can be set to an arbitrary value in accordance with an operator's preference or an operating condition, and thus a hydraulic construction machine can be provided which meets the preferences and has such improved operating performance that it can be adapted to various operating conditions in a wide range.

When the maximum output value of the function generator 63d is set to be larger than the maximum output value of the input torque control section 62, the displacement of the pump can be controlled only by the input torque control in the same manner as described above.

- THIRD EMBODIMENT -

Fig. 5 to 7 show a third embodiment. In the first and second embodiments, the standby flow rate control sections 63 and 63A are respectively independent of the LS control section 61 outputting the first target displacement θL and the input control section 62 outputting the second target displacement θO. provided, and the standby discharge flow rate is changed by the operator's operation of the control member. In the third embodiment, the standby discharge flow rate control section is omitted, and the load sensing control is inhibited when a specific operating condition is detected, thereby controlling the displacement of the hydraulic pump by the input torque limiting control. In the third embodiment, when the vehicle is running, the load sensing control and the input torque limiting control are selectively executed, and in the working operation, using the operating lever 58 together with controlling the engine speed by the accelerator pedal 6a, the input torque limiting control is executed without using the load sensing control.

The main section of the whole circuit of the hydraulic construction machine is the same as that shown in Figs. 2 and 3, and the different point is described in the main.

The control device 50 is equipped with a first control circuit section 60B as shown in Fig. 5 and a second control circuit section 80 as shown in Fig. 6. In the first control circuit section 60B shown in Fig. 5, the standby throughput control section 63 shown in Fig. 1 is omitted, a selector switch 66 is provided at a rear stage of the minimum value selection section 64, and a judgment section 67 serving as a switch control section of the switch 66 is provided.

The judging section 67 includes a comparator 67a and an AND circuit 67b and is constructed to output a high level signal when the operating lever 58 is operated while the engine speed is controlled by the accelerator pedal 6a so that the "work" is carried out. The comparator 67a outputs a high level signal when the sensor pressure Pt detected by the pressure sensor 56 is higher than a predetermined pressure set in advance (by a reference power supply 67c), and the output signal therefrom is supplied to the AND circuit 67b. The AND circuit 67b is also supplied with a signal representing the on/off state of the brake switch SW2 and a signal representing the switching state of the forward/reverse travel changeover switch SW1, and the AND circuit 67b is activated when the following four conditions are completely satisfied:

(1) The brake switch SW2 is on (high level signal is output),

(2) the forward/reverse switch SW1 is in its N position (high level signal is output),

(3) the servo pressure Pt as described above is higher than the predetermined value (high level signal is output from the comparator 67a), and

(4) The working servo pressure switch 95 is on (high level signal is output).

If the work is carried out in a vehicle-stopped state, while the rotation speed of the engine 27 is controlled by the operation of the accelerator pedal 6a, the above four conditions are completely satisfied and thus the high level signal is output from the AND circuit 671).

The selector switch 66 is switched to an a-contact point when the AND circuit 67b is in an off state (when the low-level signal is output and the vehicle is running) to select the output of the minimum value selection section 64. That is, the smaller of the first target displacement θL and the second target displacement θA is selected as the displacement instruction value θr. When the AND circuit 671) is activated (when the high-level signal is output and there is a working operation under the special operation conditions as described above), the selector switch 66 is switched to a b-contact point to select the output of the torque control section 62. That is, the second target displacement θA is selected as the displacement target instruction value θr. The selected displacement command value θr is supplied to the positioning control section 66.

The controller 50 also includes the second control circuit section 80, as shown in Fig. 6.

In the second control circuit section 80, reference numeral 81 denotes a first target speed calculation section to which a signal corresponding to the operation amount X of the accelerator lever 57a of the speed setting device 57 is supplied to determine a first target speed Nx corresponding to the operation amount X. Reference numeral 82 represents a second target speed calculation section to which the servo pressure Pt detected by the pressure sensor 56 is supplied, which indicates the operation amount of the accelerator pedal 6a, and serves to determine a second target speed Nt corresponding to the servo pressure Pt. Here, in the first target speed calculation section 81, the operation amount X and the first target speed Nx are set in such a relationship that the first target speed Nx is increased linearly with increasing displacement amount X from an idling speed Ni. In the second target speed calculating section 82, the servo pressure Pt and the second target speed Nt are set in such a relationship that the second target speed Nt is increased linearly with increasing servo pressure Pt (pedal operation amount) from the idling speed Ni. Further, the maximum value Nxmax of the first target speed Nx is set to be below the maximum speed that can be output from the engine 27, and the maximum value Ntmax of the second target speed Nt is set to be substantially equal to the maximum speed of the engine 27. Therefore, the maximum value Ntmax of the target speed Nt is larger than the maximum value Nxmax of the target speed Nx.

Of the target speeds Nx and Nt, the larger one is selected in the maximum value selection section 83 and set as the target speed instruction value Ny. In the setting control section 84, the target speed instruction value Ny is compared with the control speed Nθ indicated by the displacement amount of the control lever 271) detected by the potentiometer 55, and the stepping motor 28 is controlled so that both of them coincide with each other in accordance with the procedure shown in Fig. 7.

At step S21 in Fig. 7, the target speed instruction value Ny and the control speed Nθ indicated by the amount of movement of the governor lever are read, and the flow goes to a step S22. At step S22, the result of Nθ - Ny is stored as the differential speed A in a memory, and at step S23, it is judged whether A ≥ K based on a predetermined reference differential speed K. If the judgment at step S23 is "yes", the flow goes to a step S24, and it is judged whether the differential speed A>0. If A>0, the control speed Nθ indicated by the amount of movement of the governor lever is greater than the target speed instruction value Ny, that is, the control speed is greater than the target speed. Therefore, at step S25, a signal instructing the reverse rotation of the motor is output to the stepping motor 28 to reduce the engine speed. By this operation, the stepping motor 28 is rotated reversely and the engine speed 27 is reduced.

On the other hand, when A ≤ 0, the control speed Nθ indicated by the displacement amount of the governor lever is smaller than the target speed Ny, that is, the control speed is lower than the target speed. Therefore, at step S26, a signal indicating the forward rotation of the motor is output to increase the engine speed. By this operation, the stepping motor 28 is rotated forward and the rotational speed of the engine 27 is increased. If the judgment at step S23 is "No", the flow advances to a step S27 to output a motor stop signal so that the rotational speed of the engine 27 is maintained at a constant value. After the processes at steps S25 to S27 are executed, the flow returns to the initial step.

The operation of the third embodiment constructed in this way is described below.

When a work such as excavation is carried out by driving the front attachment, the brake switch SW2 is turned on and the forward/reverse travel changeover switch SW1 is switched to the N position. The control of the engine speed can be carried out by the accelerator lever 57a or the accelerator pedal 6a. When the control of the engine speed is carried out without using the accelerator pedal 6a, the accelerator lever 57a of the speed setting device 57 is kept at the maximum stroke position or an appropriate intermediate position below the maximum stroke. If the engine speed is controlled using the accelerator lever 57a as described above, the travel servo pressure Pt supplied to the second target speed calculating section 82 of the second control circuit section 80 is zero, so that the second target speed Nt calculated in the above calculating section 82 becomes the idling speed Ni. Further, the operation amount X of the accelerator lever supplied to the first target speed calculation section 81 is a value larger than zero, so that the first target speed Nx calculated in the above calculation section 81 becomes a value which is larger than the idling speed Ni and corresponds to the stroke position of the accelerator lever 57a. Therefore, in the selection section 83, the target speed Nx is selected as the target speed instruction value Ny, and the engine 27 is controlled to the target speed Nx. By this operation, the engine speed is controlled to be maintained at a constant value corresponding to the stroke position of the accelerator lever.

On the other hand, the following process is executed in the first control circuit section 60B of the controller 50.

In the working mode in which the engine speed is controlled by the throttle lever 57a, the brake switch SW2 is turned on and the forward/reverse travel switch SW1 is in its N position (neutral position), and the pressure switch 95 is turned on by the operation of the operating lever 58. However, the servo pressure Pt is lower than the predetermined value because the accelerator pedal 6a is not operated (conditions (1), (2), and (4) out of the above-described conditions (1), (2), (3), and (4) are satisfied, but not condition (3)), and the AND circuit 67b constituting the judging section 67 is activated. Therefore, the selector switch 66 selects the signal from the minimum value selecting section 64. That is, of the first and third target displacements θL and θA, the smaller one is selected as the displacement instruction value θr, and the displacement of the hydraulic pump 1 is controlled to be equal to the selected displacement instruction value θr.

As described above, when the "work" is performed by controlling the engine speed using the throttle lever 57a, the pressure output from the hydraulic pump 1 is controlled to be higher than the load pressure of the hydraulic motor 3 by the target differential pressure ΔPLSR when using the first target displacement θL (the load measurement control is carried out), and the input torque of the hydraulic pump 1 is controlled not to exceed the target torque Tp0 when using the second target displacement θA (the input torque limiting control is carried out).

Even when the engine speed is controlled by the accelerator pedal 6a in the vehicle running operation, the selector switch 66 is switched to the a contact since the brake switch SW2 is turned off, and the pump tilt angle is controlled with the smaller value of θL and θA.

In a working operation in which the engine speed is controlled by the operation of the accelerator pedal 6a to perform the speed control of the front attachment, the brake switch SW is turned on. The forward/reverse switch SW1 is switched to the neutral N position, and the accelerator lever 57a remains at the minimum stroke position. When the pedal 6a is depressed in this state, the accelerator operation amount X = 0 is supplied to the first target speed calculation section 81 of the second control circuit section 80, so that the first target speed Nx calculated in the calculation section 81 becomes equal to the idling speed Ni. Further, the travel servo pressure Pt corresponding to the operation amount (depression amount) of the accelerator pedal 6a is supplied to the second target speed calculation section 82, so that the first target speed Nx calculated in the calculation section 82 calculated second target speed Nt becomes a value which is higher than the idling speed Ni and corresponds to the depression amount of the accelerator pedal 6a. Therefore, the target speed Nt is selected as the target speed instruction value Ny in the selection section 83, and the speed of the engine 27 is controlled to the target speed Nt. As a result, the engine speed is controlled in accordance with the depression amount of the accelerator pedal 6a, so that the engine speed is increased as the depression amount of the accelerator pedal 6a increases, but decreases as the accelerator pedal 6a is released.

On the other hand, in the first control circuit section 60B, the following process is executed.

In the working operation in which the engine speed is controlled by the accelerator pedal 6a, the brake switch SW2 is turned on, the forward/reverse changeover switch SW1 is in its N position (neutral position), the servo pressure Pt has a value higher than the value predetermined by the operation of the accelerator pedal 6a, and the pressure switch 95 is turned on by the operation of the working lever 58. Therefore, all of the above-described conditions (1), (2), (3) and (4) are satisfied, so that the AND circuit 67b serving as the judging section 67 is actuated. By this operation, the selector switch 66 is switched to the b contact point, so that the second target displacement θA calculated in the torque control section 62 is selected as the displacement instruction value θr. The displacement of the hydraulic pump 1 is controlled to be equal to the selected displacement command value θr. That is, the load measurement control as described above is not carried out, and only the input torque limiting control is carried out. Therefore, in a range below the second target displacement determined by the input torque control, the flow rate output from the hydraulic pump 1 is increased or decreased substantially in proportion to the increase/decrease control of the engine speed described above using the accelerator pedal 6a, so that the driving of the front attachment such as the boom can be controlled at a desired speed.

If the selector switch 66 shown in Fig. 5 is designed so that it can be switched at will by the operator, the optional use of the load sensing control and the input torque control based on the operator's judgment. As described in addition in the first and second embodiments, even in a case where the control system is adopted in which the load sensing control is disabled and the pump displacement is controlled by the input torque control based on the operator's judgment, the same effect as in the third embodiment can be obtained.

In the above embodiment, the existing accelerator pedal 6a is also used as the operating means for the engine speed control. However, the operating means is not limited to the accelerator pedal 6a, and may be another operating member (preferably a pedal). Further, in the above embodiment, the traveling hydraulic motor 4 and the working hydraulic cylinder 20A are provided as the hydraulic operating means. However, the hydraulic operating means may be constructed to have a plurality of hydraulic working cylinders 20A and a hydraulic motor for rotating the upper swing body of the vehicle. Further, the hydraulic working cylinder 20A is not limited to a cylinder for driving a boom, and may be a cylinder for driving the arm or the bucket.

Fig. 8 is a detailed block diagram of the torque control section 62 of the first to third embodiments described above.

The control speed Nθ corresponding to the governor lever position detected by the potentiometer 55 is supplied to a reference speed calculation section 162a and a torque calculation section 162b. The reference speed calculation section 162a calculates a speed measurement reference speed Ns in accordance with the input control speed Nθ based on a characteristic shown in the figure. The reference speed Ns is increased as the control speed Nθ increases. In addition, the torque calculation section 162b calculates the target torque Tro in accordance with the input control speed Nθ based on a characteristic shown in the figure. An adder 162c serves to calculate the deviation ΔN (= Nr - Ns) between the actual engine speed Nr and the above-described reference speed Ns, and a correction torque calculation section 162d serves to calculate a correction torque ΔT in accordance with the speed deviation ΔN based on a characteristic curve shown in the figure. The correction torque is positive when the speed deviation AN is positive and is negative when the speed deviation ΔN is negative. In addition, the correction torque ΔT increases with an increase in ΔN.

A function generator 162e outputs a signal representing "0" when the control speed Nθ is lower than a predetermined value and "1" when the control speed Nθ is equal to or greater than the predetermined value, and the signal is multiplied by the correction torque ΔT in a multiplier 162f. That is, the correction torque ΔT is effective only when the control speed is equal to or greater than the predetermined value. ΔT is added to the target torque Tro described above in the adder 162g, and the result of this addition is output as the target torque instruction value Tpo. The target displacement θA is calculated based on the target torque instruction value Tpo.

According to the above-described input torque control section, when the engine is in a tight torque, the corrector torque ΔT becomes positive so that the target torque instruction value Tpo is increased. On the other hand, when the engine is in an over-torque state, the corrector torque ΔT becomes negative so that the target torque instruction value Tpo is caused to decrease. Therefore, the target speed can be approximated to a calculated nominal speed, and thus the torque can be effectively set.

With the input torque control section, no problem occurs if the operation is carried out by maintaining an engine speed at a constant value using the accelerator lever 57a, but the following problem occurs if the operation of the vehicle is carried out by increasing/decreasing the engine speed using the accelerator pedal 6a.

When the accelerator pedal 6a is rapidly depressed to a full throttle position, the governor lever 27b is immediately operated to a full throttle position by the stepping motor 28 so that the control speed Nθ is increased, and the speed measuring reference speed is also immediately increased. However, when the amount of operation of the governor lever reaches the maximum value, there is a Time delay until the actual engine speed is increased to its maximum value. The speed delay ΔN is negative during this time delay, and the correction torque ΔT is also negative. Therefore, the target torque command value Tpo is decreased so that the displacement of the hydraulic pump decreases, and the increase in speed is impaired.

In this connection, an embodiment for improving the rise characteristic of the engine speed as described above is shown in Fig. 9.

Fig. 9 shows the input torque control section 162 of the first control circuit section 60 within the controller 50. The same elements as in Fig. 8 are denoted by the same reference numerals, and the different points will be mainly described.

The input torque control section 132 is equipped with a torque calculation section 162k in parallel with the torque calculation section 162d. The torque calculation section 162k has a characteristic different from that of the torque calculation section 162d. The output value of each torque calculation section is supplied to the selection section 162h. In the torque calculation section 162k, the correction torque ΔT becomes positive and increases with an increase of ΔN when the speed deviation ΔN is positive. On the other hand, when ΔN is negative, ΔT is zero. That is, only control for increasing the input torque is carried out.

The input torque control section 162 includes an AND circuit 162i, and the AND circuit 162i is supplied with a signal representing an on/off state of the brake switch SW2, a signal representing an operation position of the forward/reverse changeover switch SW1, and an output of a function generator 162j. The function generator 162j outputs "1" when an operation amount of the accelerator pedal 6a is equal to or greater than a predetermined amount, and outputs "0" when it is smaller than the predetermined amount, and is supplied with a signal Pt representing the operation amount of the accelerator pedal 6a, which is an output of the pressure sensor 56. Therefore, when the following conditions are met, that is, when the vehicle is running, the AND circuit 162i is activated to output a high level signal to a selector switch 162h. and the selector switch 162h selects the torque calculation section 162k:

(1) The brake switch SW2 is in an off state (the switch SW2 outputs a low-level signal),

(2) The forward/reverse changeover valve 8A is switched to a position other than the N position (the forward/reverse changeover switch SW1 outputs a low level signal), and

(3) The accelerator pedal 6a is depressed to an extent greater than the predetermined extent (the function generator 162j outputs a high level signal).

According to the input torque control section 162 thus constructed, when the accelerator pedal 6a is depressed to the full throttle position in the running operation, the control speed Nθ, which is a detection value of the governor lever position, becomes the maximum value, and along with the maximum value of the control speed, the speed measuring reference speed Ns also becomes the maximum value. A time delay is required until the actual engine speed reaches a value corresponding to the maximum value of the governor lever, and during this time delay, the speed deviation ΔN is negative. However, since the torque calculation section 162k is selected during the running of the vehicle, the correction torque ΔT is not negative and is zero even if the speed deviation ΔN becomes negative. Therefore, the target torque command value Tpo is not reduced, and the displacement of the hydraulic pump 1 is not reduced, so that the acceleration performance for vehicle propulsion can be improved more in this case than in the case of using the input torque control section as shown in Fig. 8.

Further, since the torque calculation section 162d is selected by the selector switch 162h in a non-driving operation such as an excavation work, as in the conventional input torque control section, the pump displacement is reduced to reduce the engine input torque when the engine does not have excess torque, so that undesirable stalling of the engine can be prevented.

Such a structure for ensuring acceleration performance can be used both for turning the upper swing body of the vehicle and during driving.

The hydraulic control device of the invention described above is effectively applied in particular to various hydraulic construction machines equipped with a diesel engine, such as a hydraulic wheel excavator, a hydraulic crawler excavator, a hydraulic crawler crane, a wheel loader, etc.

Claims (10)

1. Hydraulic control system for a hydraulic construction machine with: a hydraulic variable displacement pump (1) driven by a prime mover (27), a first hydraulic actuating device (4) operated with oil delivered by the hydraulic pump, a first control valve (2) which is mounted between the hydraulic pump (1) and the first hydraulic actuating device (4) and serves to control the supply of the pressurized oil to the first hydraulic actuating device, a first determination device (61) for determining a first target adjustment (ΘL) so that a discharge pressure of the hydraulic pump is maintained by a predetermined target value at a value above the load pressure of the first hydraulic actuating device, a second determining device (62) for determining a second target adjustment (ΘA) with which an input torque is limited based on the output pressure of the hydraulic pump, and a displacement control device (65) for controlling the displacement of the hydraulic pump so that the adjustment equals the first and/or the second target adjustment, characterized in that the hydraulic control system further includes a limiting device (63) with which a limit can be imposed on a signal representing the first target adjustment determined and output by the first determining device, the limiting device containing the following: a limit signal output device (63a) for outputting a limit signal (Θo) representing a value that is greater than the minimum value of the first target displacement (ΘL) output by the first determining device, a maximum value selection device (63c) for comparing the value represented by the limit signal with the first target adjustment output by the first determination device and for selecting and outputting the larger of these values, and a limiting operation instructing signal output device (63b; 63d) for outputting a limiting operation instructing signal which causes the operation of the limiting signal output device, and wherein the adjustment is controlled so that it is equal to the smaller of the target adjustment selected by the maximum value selection device and the second target adjustment. 2. Hydraulic control system according to claim 1, in which the device for outputting a signal instructing a limiting operation is operated by an intervention of the operator. 3. Hydraulic control system according to claim 1, wherein the device for outputting a limiting operation instructing Signal (63b; 63d) is operated by the operation of an operating element of the hydraulic actuating device (4). 4. Hydraulic control system according to one of claims 1 to 3, in which the device for outputting a signal instructing a limiting operation comprises a switch (SW3) which is operated by an operator. 5. Hydraulic control system according to one of claims 1 to 3, wherein the limiting operation instructing signal output device (63d) outputs at least two types of limiting operation instructing signals and the limiting signal output device outputs a target displacement (Θo) whose value corresponds to an input limiting operation instructing signal. 6. Hydraulic control system according to claim 5, wherein the device for outputting a signal instructing a limiting operation includes an operating element (SW3) for outputting at least two types of signals (SS) instructing a limiting operation, according to the manner in which the operating element is operated by the operator. 7. Hydraulic control system according to one of claims 1 to 3, wherein the hydraulic construction machine further contains: a first operating device (57; 81) which is operated to maintain the speed of the prime mover at any speed, a second operating device (56; 82) which is operated to control the speed of the prime mover (27) and is reset to an initial low speed position when the operating force is released, a speed control device (83, 84; 28) for controlling the speed of the engine in accordance with the first and second actuating devices, a second hydraulic actuating device (21) which is operated with the oil delivered by the hydraulic pump, and a second control valve (20A) mounted between the hydraulic pump and the second hydraulic actuator and serving to control the pressurized oil supplied to the second hydraulic actuator, and wherein the limiting operation instructing signal output device is a checking device (67) that outputs a selection instructing signal representing the selection of the second target displacement (ΘA) based on the discrimination of a state in which the first hydraulic actuator (4) is operated while the rotational speed of the engine is controlled by the second operating device. 8. Hydraulic control system according to one of claims 1 to 7, in which the first determination device (61) for calculating the deviation between a predetermined target differential pressure and a detected differential pressure detects the differential pressure between the pressure in a line for connecting the first control valve (2) to the hydraulic pump (1) and the pressure in a line for connecting the first control valve (2) to the first hydraulic actuating device (4) and calculates the first target adjustment on the basis of this deviation, and wherein the second determining device (62) detects a deviation between an actual speed (Nr) of a diesel engine serving as a prime mover and a speed determined by the position a control speed specified by a governor lever (27b), a target torque (�Theta;A) at which the diesel engine is not stalled is calculated on the basis of the detected deviation, and the discharge pressure of the hydraulic adjusting pump is detected in order to calculate the second target adjustment on the basis of the reciprocal of the detected discharge pressure and the target torque. 9. Hydraulic control system according to claim 8, in which the second determination device (62) contains a device for checking the driving operation, and in which in driving operation the solid torque is only subjected to a correction to increase the target torque if the actual speed is higher than the control speed, and in which in non-driving operation the target torque is subjected to a correction to increase the target torque if the actual speed is higher than the control speed and to a correction to reduce the target torque if the actual speed is lower than the control speed. 10. Hydraulic control system for a hydraulic construction machine with: a hydraulic variable displacement pump (1) driven by a prime mover (27), a first hydraulic actuating device (4) operated with oil delivered by the hydraulic pump, a first control valve (2) which is mounted between the hydraulic pump (1) and the first hydraulic actuating device (4) and serves to control the supply of the pressurized oil to the first hydraulic actuating device, a first determining device (61) for determining a first target adjustment (�Theta;L) so that a discharge pressure of the hydraulic pump is adjusted by a predetermined target value to a value above the load pressure of the first hydraulic actuating device, a second determining device (62) for determining a second input torque adjustment (ΘA) with which an input torque is limited based on the output pressure of the hydraulic pump, and a displacement control device (65) for controlling the displacement of the hydraulic pump so that the adjustment equals the first and/or the second solenoid adjustment, characterized in that the hydraulic control system further includes a limiting device with which a limit can be imposed on a signal representing the first target adjustment determined and output by the first determining device, the limiting device having the following a minimum value selection device (64) with which the smaller of the first target adjustment (ΘL) output by the first determination device (61) and the second target adjustment (ΘA) output by the second determination device (62) can be selected, a selection device (66) for selecting the target adjustment selected by the minimum value selection device or the second target adjustment, a selection instructing signal output device (67) for outputting a selection instructing signal for instructing the selection of the output of the minimum value selecting device or the second target adjustment as the target adjustment to be selected in the selecting device (66).